I find this to be the case in general (not always) but wondering why. Presumably a used stamp looked just as fresh and bright as a mint stamp right after it was used, and presumably it was cared for in a collection similarly in most cases. So why the faded colour and toning on so many?
Is it the oxidization of the saliva and gum over time?
Could be all kinds of reasons: exposure to sunlight dirt the soaking process
In general I think a used stamp will change hands (and be handled) many more times than a mint stamp. Mint stamps are bought by collectors and then put into albums -- used stamps go into kiloware, get sorted multiple times even before being soaked, then sorted some more ...
Not to mention wear and tear. Used stamps have fulfilled their postal obligations and between getting tossed into dirty mailboxes, carrier bags, sorting systems, and every other thing a cover encounters in its journey they will get dirty and worn.